Lubricants are widely used to reduce friction between surfaces of moving parts and thereby reduce wear and prevent damage to the surfaces and parts. Lubricants are composed principally of a base stock and a lubricant additive. The base stock is generally a relatively high molecular weight hydrocarbon. In applications where there is a large amount of pressure applied to moving parts, lubricating compositions composed only of hydrocarbon base stock tend to fail and the parts become damaged. This problem has been addressed by adding materials (i.e., lubricant additives) to the lubricating composition to increase high pressure performance. Such additives are called "extreme pressure additives." Examples of extreme pressure additives are sulfurized sperm whale oil and sulfurized jojoba oil. There is a continuing need in the art to find alternate extreme pressure additives because sperm whale oil is no longer available due to an international ban and jojoba oil is expensive and in short supply.
Extreme pressure additives prevent destructive metal-to-metal contact in lubrication at high pressure and/or temperature such as that found in certain gear elements in automotive vehicles and various industrial machines where high pressure can cause a film of lubricant to rupture. Extreme pressure/anti-wear lubricants should have good lubricity, good cooling properties, high film strength, good load bearing abilty, and miscibility with the usual types of base oils.
To make lubricants, such as motor oils, transmission fluids, gear oils, industrial lubricating oils, metal working oils, etc., one starts with a lubricant grade of petroleum oil from a refinery, or a suitable polymerized petrochemical fluid. Into this "base stock" is blended small amounts of specialty chemicals that enhance lubricity, inhibit wear and corrosion of metals, and retard damage to the fluid from heat and oxidation.
Anti-wear agents, extreme pressure agents and friction modifiers have been developed that are generally organic or organometallic compounds containing halogens, sulfur, phosphorus, or a combination of the three. Halogens have noted low-temperature metal-coating activity but can cause serious corrosion problems at the higher operating temperatures of modern vehicles or industrial machinery and have environmental problems upon disposal. Manufactures have, therefore, switched to derivatives of sulfur and phosphorus for lubricant additives.
Before 1972, lubricant additives were based on raw and chemical derivatives of sperm whale oil, a mono-ester of monounsaturated fatty acid chains. Replacement additives include phosphorized lard oils, sulfurized polyisobutylene and moderate molecular weight polymers. These additives have met with limited success. Better lubricating properties (i.e., friction and reduced wear) have been achieved with a natural wax ester, such as jojoba oil. Lubricant additives using jojoba oil have been described in U.S. Pat. No. 4,873,008, the disclosure of which is incorporated by reference herein. Jojoba oil suffers from limited availability and high cost.
Synthetic wax esters can be made by esterifying an unsaturated fatty acid and an unsaturated fatty alcohol. Synthetic wax ester can be sulfurized. Sulfurized wax esters often display excellent lubricating properties. However, the cost of a process to create and isolate a synthetic wax ester is extremely high and comparable with the cost of natural wax ester.
One solution to this problem is described in U.S. Pat. No. 4,970,010, the disclosure of which is incorporated by reference herein. This patent describes a group of sulfurized derivatives of triglyceride vegetable oil that achieve acceptable lubricating properties. However, processing costs are still relatively high because this process requires the presence of at least 25% wax ester and preferably, 50% wax ester. For practical applications, synthetic wax esters have to be derived from natural vegetable triglycerides, such as rapeseed oil or corn oil. Cost-adding conversion steps to a synthetic wax ester make synthetic wax esters relatively uneconomical for use as lubricant additives.
Liquid wax esters are formed by forming an ester bond between the functional groups of an unsaturated fatty acid and an unsaturated fatty alcohol. Liquid wax esters have been made from triglyceride rapeseed oil, such as a high erucic acid rapeseed (HEAR) oil by a complex and expensive process, such as is described in Bell, U.S. Pat. No. 4,152,278. HEAR oil is a triglyceride in its native form. Synthetic wax ester made from HEAR oil is a substitute for sperm whale oil or a natural wax ester, such as jojoba oil.
Synthetic wax esters can be made into phosphorous or sulfurized derivatives to improve friction, wear and extreme-pressure properties of a fluid. For example, sulfurized vegetable oil wax esters are described in U.S. Pat. No. 4,152,278 and phosphite adducts of synthetic vegetable oil wax esters are described in U.S. Pat. No. 4,970,010.
Although the supply of HEAR is more stable than the supply and availability of jojoba oil, the process of transforming a triglyceride oil into a mono-ester form is a difficult and expensive process with little, if any, cost advantage over jojoba oil. Thus, there is a need in the art to be able to use a vegetable triglyceride oil directly as a lubricant additive or as a derivative to eliminate the expensive conversion steps into a synthetic wax ester and retain the advantage of low cost and availability.
Triglyceride vegetable oils, such as HEAR, contain 10%-25% polyunsaturated fatty acids and are rich in longer chain (20-24 carbon atom) fatty acids. Dienic (two double bonds) fatty acids and trienic (three double bonds) fatty acids in a triglyceride oil are more reactive than monoenic (single double bond) fatty acids. Double bonds in a vegetable oil triglyceride, when used directly in a high temperature oxidizing environment, such as a lubricant additive, are attacked by oxygen and heat which causes the triglyceride to darken, thicken and lose solubility within lubricating oil base stocks. These undesirable properties limit the usefulness of triglyceride vegetable oils for lubricant additives. Therefore, there is a further need in the art to find an inexpensive processing means to improve the lubricating properties and characteristics of triglyceride oils for use as lubricant additives. This invention was made to satisfy those needs.